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2 I I I I I I I I I I I I I I I I I I I Rationale The purpose of this study is to determine the feasibility of using a non-lethal stomach pump to assess food habits of humpback chub in the Grand Canyon. Roundtail chub captured from the Colorado River in Colorado near the Utah state line were used as a surrogate species for the analysis. The objectives of this study were to (1) evaluate the efficiency of stomach flushing, and (2) determine if damage occurs to the pharyngeal mill of processed fish. Stomach content analysis is critical in characterizing the life history and ecology of the Colorado River Basin native species. In the Grand Canyon, food habits of humpback chub, combined with food availability information from drift and benthic samples, are assessed to determine the impact of dam operations on the availability of food sources. Sampling humpback chub during changes in flow will enable us to decide if changes in behavior (i.e. additional movement) are induced by greater food availability or changes in habitat Leibfried (1988) found that rainbow trout below Glen Canyon Dam ingest large quantities of the green algae Cladophera glomerata. deriving nutritional benefit through digestion of lipid-rich diatoms epiphytic on the algae. It is important to know if humpback chub exhibit similar feeding strategies since Cladophera production is closely linked to stream flow and, hence, dam operations. Feeding habits are an important aspect in the study of aquatic ecosystems. Often, food habit studies require sacrificing large numbers of fish for stomach extraction and examination. In systems with low fish populations, or in the case of endangered or threatened species, removal of fish can seriously deplete the population. In a review of various methods for removing stomach contents from live fish, Hyslop (1980) concluded stomach pumping to be the most generally applicable. Fish species which have been tested effectively include a variety of salmonids, centrarchids, ictalurids, percids, and esocids (Meehan and Miller 1978, Swenson and Smith 1973, Seaburg and Moyle 1964). In all cases, the removal of stomach contents with pumps has not been injurious to the fish. Meehan and Miller (1978) flushed the stomachs of juvenile salmonids and found no significant difference in survival between treated and control fish after 30 days. Gengerke et al (1973) reported pumping 864 stomachs of nine species of fish with no resultant mortality. There is little information, however, on mortality rates of stomach-pumped cyprinids. Specifically, no one has assessed the potential damage inflicted by the stomach pump tube to the pharyngeal mill of cyprinids. Damage to the pharyngeals could result in impairment of feeding and represent a serious threat to the welfare of the fish. Another advantage of the stomach pumping technique is its efficiency. Fish sacrificed by Gengerke et a1. (1973) following stomach flushing contained no food items in their stomachs. Prey items recovered from the stomachs of 155 salmonids (rainbow trout, cutthroat trout, and coho salmon) comprised 93% of the total prey consumed by number, and 85% by weight (Meehan and Miller 1978); the stomach pump effectively flushed out adult and larval macroinvertebrates of all sizes, as well as small crayfish, sculpins, snail shells, stones, and large wood pieces. The removal of stomach contents of humpback chub with stomach pumps has presented a unique problem, however. Pilot studies in the Grand Canyon showed that material flushed from the stomachs of chubs was evacuated through the vent of the fish, rather than being flushed out the mouth as is the usual case (BIO/WEST, unpublished data). This is likely due to the relative straightness of the S-shaped gut of the humpback chub, compared to the highly convoluted gut of salmonids, centrarchids, and esocids. Also, the absence of a pyloric sphincter in chubs does not allow for backpressure to develop in the stomach, and the gut contents are flushed through the anal vent. The efficiency of stomach flushing through the vent has never been tested.